1. Field of the Invention
The present invention relates to allophanate group-containing polyisocyanates which not only have excellent compatibility with aldimine co-reactants, but are also useful for improving the compatibility of other polyisocyanates with aldimine co-reactants, and to their use in coating compositions.
2. Description of the Prior Art
Coating compositions based on mixtures of polyisocyanates and aldimines are known. One of the disadvantages of these compositions is the compatibility between the polyisocyanate and the aldimine, which is necessary to obtain clear, high gloss coatings. Depending upon the particular co-reactants, the mixtures range from totally incompatible systems to those which exhibit a minor amount of cloudiness. The former are totally unsuitable for the production of coatings, while the latter result in coatings which are either hazy in appearance or suffer from a lack of gloss.
The compatibility problem between polyisocyanates and aldimines is addressed in U.S. Pat. Nos. 5,466,771; 5,516,873; and 5,523,376. U.S. Pat. No. 5,466,771 describes certain allophanate-modified polyisocyanates containing isocyanurate groups that possess improved compatibility with aldimines. Similarly U.S. Pat. No. 5,523,376 describes certain uretdione-modified polyisocyanates that possess improved compatibility with aldimines. Finally, U.S. Pat. No. 5,516,873 describes the use of polyaspartic acid esters to improve the compatibility between polyisocyanates and aldimines.
While it is possible to blend the polyisocyanates described in U.S. Pat. Nos. 5,446,771 and 5,523,376 with aldimines to obtain coating compositions that are suitable for the production of acceptable coatings, there is a need to further improve their compatibility with aldimines, such as the aIdimine based on 1,6-hexamethylene diamine (HDA).
Accordingly, it is an object of the present invention to provide polyisocyanates that are not only compatible with aldimines, such as the HDA aldimine, such that the resulting coating compositions can be used to prepare coatings having a high gloss, but can also be blended with incompatible polyisocyanates to improve their compatibility with aldimines.
This object may be achieved with the polyisocyanates according to the present invention, which contain allophanate groups, are substantially free of urethane groups, have a low viscosity and are prepared by reacting an aliphatic polyisocyanate with a mixture of monohydroxy compounds and polyhydric alcohols, preferably linear or branched, aliphatic monohydric and polyhydric alcohols.
Processes for the production of polyisocyanates containing allophanate groups are described in British Patent 994,890; U.S. Pat. Nos. 3,769,318, 4,160,080, 4,177,342 and 4,810,820; and German Offenlegungsschrift 4,416,321. While some of these patents disclose that mixtures of monoalcohols and polyhydric alcohols may be used to prepare polyisocyanates containing allophanate groups and/or contain examples describing polyisocyanates prepared from mixtures of monoalcohols and polyhydric alcohols, none of these patents are directed to improving the compatibility between polyisocyanates and HDA aldimines and do not recognize the particular requirements that are necessary to achieve this compatibility.
Polyisocyanates containing allophanate groups and prepared from a mixture of monoalcohols and polyhydric alcohols are described in certain examples of U.S. Pat. No. 4,810,820. The percentage of urethane groups converted to allophanate groups, which is set forth in the tables, was obtained by determining the reduction in the NCO content after urethane formation. This calculation is based on the assumption that every isocyanate group that was consumed reacted with a urethane group to form an allophanate group. However, this assumption is not accurate. For example, isocyanate groups can react with other isocyanate groups to form uretdione groups or isocyanurate groups. The examples set forth hereinafter demonstrate that contrary to the polyisocyanates of the present invention, the products of U.S. Pat. No. 4,810,820 are not substantially free of urethane groups.